Inkjet printers have printheads that operate a plurality of inkjet ejectors from which liquid ink is expelled. The ink may be stored in reservoirs located within cartridges installed in the printer, or the ink may be provided in a solid form and then melted to generate liquid ink for printing. In these solid ink printers, the solid ink may be in either pellets, ink sticks, granules or any other forms. The solid ink pellets or ink sticks are typically placed in an “ink loader” that is adjacent to a feed chute or channel. A feed mechanism moves the solid ink sticks from the ink loader into the feed channel and then urges the ink sticks through the feed channel to a heater assembly where the ink is melted. In some solid ink printers, gravity pulls solid ink sticks through the feed channel to the heater assembly. Typically, a heater plate (“melt plate”) in the heater assembly melts the solid ink impinging on it into a liquid that is delivered to a printhead for jetting onto a recording medium.
A typical inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be recording media or it may be a rotating intermediate image receiving member, such as a print drum or belt. In the printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual drops at particular locations of a pixel array defined for the receiving medium. The locations are sometimes called “drop locations,” “drop positions,” or “pixels.” Thus, the printing operation can be viewed as the filling of a pattern of drop locations with drops of ink.
Some inkjet printheads, such as phase change inkjet printheads, utilize inks that have melting points of 80° C. and higher. With many of these inks, optimal jetting occurs at significantly higher temperatures, such as 100°-120° C. and above. Consequently, during printing the inkjets and other printhead components must be maintained at or above these elevated jetting temperatures. The temperature of the ink reservoirs supplying liquid ink to the inkjets must also be maintained at or near the required jetting temperatures.
Prolonged use of an inkjet printhead at elevated temperatures can alter printhead performance and accelerate thermal stress or aging of the printhead components. Thermal aging, also known as drift, can result in image degradation due to performance variations. For example, the drop mass of ejected ink drops can vary as the printhead components are thermally conditioned over time. Variations in drop mass from nozzle to nozzle of a printhead or from printhead to printhead in a multiple printhead system may result in result in banding or streaking of a printed image, blurred edges to lines or shapes due to positional errors resulting from drift, or low intensity in solid colors.
To reduce ink drop mass variations due to thermal aging of the printheads of an inkjet printer, previously known systems implemented an open loop process in which a controller altered the voltage level of the firing signals for the printhead over time at a predefined rate that was designed to compensate for the drift of a generic printhead. The variability of the drift behavior between different printheads in a printer, however, may be significant and may be in opposite directions. Therefore, adjusting the driving voltages of the printheads in this manner may eventually result in printheads outputting drops at different drop masses.